. Effect of sterol carrier protein-2 gene ablation on HDLmediated cholesterol efflux from cultured primary mouse hepatocytes. Am J Physiol Gastrointest Liver Physiol 299: G244-G254, 2010. First published April 15, 2010 doi:10.1152/ajpgi.00446.2009.-Although HDL-mediated cholesterol transport to the liver is well studied, cholesterol efflux from hepatocytes back to HDL is less well understood. Real-time imaging of efflux of 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-amino)-23,24-bisnor-5-cholen-3â€-ol (NBD-cholesterol), which is poorly esterified, and [ 3 H]cholesterol, which is extensively esterified, from cultured primary hepatocytes of wild-type and sterol carrier protein-2 (SCP-2) gene-ablated mice showed that 1) NBD-cholesterol efflux was affected by the type of lipoprotein acceptor, i.e., HDL3 over HDL2; 2) NBD-cholesterol efflux was rapid (detected in 1-2 min) and resolved into fast [half time (t1/2) Ï 2.4 min, 6% of total] and slow (t 1/2 Ï 26.5 min, 94% of total) pools, consistent with protein-and vesicle-mediated cholesterol transfer, respectively; 3) SCP-2 gene ablation increased efflux of NBD-cholesterol, as well as [3 H]cholesterol, albeit less so due to competition by esterification of [ 3 H]cholesterol, but not NBD-cholesterol; and 4) SCP-2 gene ablation increased initial rate (2.3-fold) and size (9.7-fold) of rapid effluxing sterol, suggesting an increased contribution of molecular cholesterol transfer. In addition, colocalization, double-immunolabeling fluorescence resonance energy transfer, and electron microscopy, as well as cross-linking coimmunoprecipitation, indicated that SCP-2 directly interacted with the HDL receptor, scavenger receptor class B type 1 (SRB1), in hepatocytes. Other membrane proteins in cholesterol efflux [SRB1 and ATP-binding cassettes (ABC) A-1, ABCG-1, ABCG-5, and ABCG-8] and several soluble/vesicle-associated proteins facilitating intracellular cholesterol trafficking (StARDs, NPCs, ORPs) were not upregulated. However, loss of SCP-2 elicited twofold upregulation of liver fatty acid-binding protein (L-FABP), a protein with lower affinity for cholesterol but higher cytosolic concentration than SCP-2. Ablation of SCP-2 and L-FABP decreased HDL-mediated NBD-cholesterol efflux. These results indicate that SCP-2 expression plays a significant role in HDL-mediated cholesterol efflux by regulating the size of rapid vs. slow cholesterol efflux pools and/or eliciting concomitant upregulation of L-FABP in cultured primary hepatocytes. confocal imaging; bile; scavenger receptor B1 EXCESS CHOLESTEROL IS PRIMARILY removed from the body via the liver into bile, and ÏŸ95% of excreted cholesterol originates from lipoproteins, primarily HDL (9). HDL removes unesterified (free) cholesterol from tissues, transports free (as well as esterified) cholesterol to the liver, and binds to scavenger receptor class B type 1 (SRB1) at hepatocyte basolateral plasma membranes (9). Hepatic clearance of HDL free cholesterol from blood is rapid [half time (t 1/2 ) Ï 3 min] (15, 31), consistent with very ra...